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Unit 1-Semiconductors & Rectifiers

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Unit 1-Semiconductors & Rectifiers www.Vidyarthiplus.com QUESTION BANK DEPARTMENT: ME YR/ SEM:II/ III SUB CODE: ME2255 SUB NAME: ELECTRONICS & MICROPROCESSORS UNIT 1-SEMICONDUCTORS & RECTIFIERS PART A (2 Marks) 1. Define Rectification. (AUC MAY 2012) A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Rectifiers have many uses, but are often found serving as components of DC power supplies and high-voltage direct current power transmission systems. 2. Define voltage regulation. (AUC MAY 2012) voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that result in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage. 3. Draw the circuit of zener voltage regulator. (AUC MAY 2010) ME2255/Electronics& Microprocessors Department of ME 1 www.Vidyarthiplus.com www.Vidyarthiplus.com 4. A half wave rectifier having a resistance load of 1KΩ rectifies an alternating voltage of 325V peak value and the diode has a forward resistance of 100Ω. Calculate its DC power output. (AUC MAY 2010) 5. What is diffusion current? (AUC MAY 2011) In a non uniformly doped semi conductor charge carriers have a tendency to move from a region of higher concentration to a region of lower concentration. The flow of current due to this process is called diffusion current. 6. Draw the circuit of bridge rectifier with input & output waveforms. (AUC MAY 2011) 7. What is a PN junction. (AUC NOV 2010, NOV 2011) The junction created when a p type & n type semiconductors joined together is called pn junction. The device formed in such a way is called pn junction diode. 8. Define rectifier. (AUC NOV 2010) Same as Q1 9. What is a rectifier? What are its types. (AUC NOV 2011) Same as Q1 The types of rectifiers are, Half wave rectifier Full wave rectifier Bridge rectifier 10. What is meant by dynamic resistance of diode? Dynamic resistance of a diode can be defined as the ratio of change in voltage across the diode to the change in current through the diode. ME2255/Electronics& Microprocessors Department of ME 2 www.Vidyarthiplus.com www.Vidyarthiplus.com r = V / I Where r - Dynamic resistance of a diode V - change in voltage across the diode I - change in current through the diode 11. Differentiate between zener breakdown and avalanche breakdown. Zener breakdown occurs in a reverse biased junction which gives a constant output voltage. Avalanche breakdown does not provide a constant output voltage. This constant voltage from a zener diode can be used as a reference voltage for many regulators. 12. Define Knee voltage of a diode. Knee voltage of a diode is defined as a breakover voltage above which the forward current increases abruptly. This voltage is also called as breakdown voltage of a diode. It is 0.3 for Ge and 0.7 for Si. 13. What is peak inverse voltage? The maximum reverse-bias potential that can be applied before entering the Zener region is called the peak inverse voltage (referred to simply as the PIV rating) or the peak reverse voltage (denoted by PRV rating). Peak inverse voltage is defined as the maximum reverse voltage that a diode can be subjected to operate in a reverse region so that the diode does not get damaged due to this reverse voltage. 14. What is meant by doping in a semiconductor? The method of adding impurities to a semiconductor to make them conduct is called doping. It is the process of adding trivalent, pentavalent impurity to the semiconductor which gives rise to P type and N type semiconductor. 15. Give the equation for diode current under reverse bias. eVD/nVT ID=IO [ -1] 16. What is diffusion capacitance? Charge carriers flowing out of a depletion region, which is widened when the junction is reverse biased, there is a large reverse current at first and which slowly decreases to the level of reverse saturation current. The effect is like a discharging of a capacitor called as diffusion capacitance. 17. How do the transition region width and contact potential across a PN junction vary with the applied bias voltage? ME2255/Electronics& Microprocessors Department of ME 3 www.Vidyarthiplus.com www.Vidyarthiplus.com The width of the PN junction is widenened for a reverse biased junction and narrows for a forward biased junction. 18. What is an ideal diode? An ideal diode is one which offers zero resistance when forward biased and infinite resistance when reverse biased. 19. Compare ideal diode as a switch. An ideal diode when forward biased is equivalent a closed (ON) switch and when reverse biased, it is equivalent to an open (OFF) switch. 20. State the mathematical equation which relates voltage applied across the PN junction diode and current flowing through it. 21. Define knee/cut-in/threshold voltage of a PN diode. It is the forward voltage applied across the PN diode below which practically no current flows. 22. What is the effect of junction temperature on cut-in voltage of a PN diode? Cut-in voltage of a PN diode decreases as junction temperature increases. 23. What is the effect of junction temperature on forward current and reverse current of a PN diode? For the same forward voltage, the forward current of a PN diode increases and reverse saturation current increases with increase in junction temperature. 24. Differentiate between breakdown voltage and PIV of a PN diode. The breakdown voltage of a PN diode is the reverse voltage applied to it at which the PN junction breaks down with sudden rise in reverse current. Whereas, the peak inverse voltage (PIV) is the maximum reverse voltage that can be applied to the PN junction without damage to the junction. 25. Differentiate avalanche and zener breakdowns. Avalanche Breakdown 1. Breakdown occurs due to heavily doped junction and applied strong electric field. 2. Doping level is high. 3. Breakdown occurs at lower voltage compared to avalanche breakdown Zener Breakdown ME2255/Electronics& Microprocessors Department of ME 4 www.Vidyarthiplus.com www.Vidyarthiplus.com 26. Draw the V-I characteristics of an ideal diode. 26. Differentiate between drift and diffusion currents. Drift Current 1. It is developed due to potential gradient. 2. This phenomenon is found both in metals and semiconductors Diffusion Current 1. t is developed to charge concentration gradient. 2. It is found only in semiconductors. 27. Define transition capacitance of a diode. Transition Capacitance (CT) or Space-charge Capacitance: When a PN- junction is reverse-biased, the depletion region acts like an insulator or as a dielectric. The P- and N-regions on either side have low resistance and act as the plates. Hence it is similar to a parallel-plate capacitor. This junction capacitance is called transition or space-charge capacitance (CT). It is given by Where, A = Cross-sectional area of depletion region. D = Width (or) thickness of depletion region. Its typical value is 40 pF. ME2255/Electronics& Microprocessors Department of ME 5 www.Vidyarthiplus.com www.Vidyarthiplus.com Since the thickness of depletion layer depends on the amount of reverse bias, CT can be controlled with the help of applied bias. This property of variable capacitance is used in varicap or varactor diode. This capacitance is is voltage dependent PART B (8, 16Marks) 1. Draw & Explain the circuit of full wave rectifier. (AUC MAY 2012, NOV 2011) Full wave Rectifier using a centre tapped transformer: A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both half cycles of the applied ac voltage. In order to rectify both the half cycles of ac input, two diodes are used in this circuit. The diodes feed a common load RL with the help of a center-tap transformer. A center-tap transformer is the one which produces two sinusoidal waveforms of same magnitude and frequency but out of phase with respect to the ground in the secondary winding of the transformer. The full wave rectifier is shown in the figure below. ME2255/Electronics& Microprocessors Department of ME 6 www.Vidyarthiplus.com www.Vidyarthiplus.com Voltage Waveforms of a center tapped full wave rectifier Operation: During the positive half cycle of the input, the anode of D1 becomes positive and the anode of D2 becomes negative. Hence, D 1 conduct and D2 does not conducts. The load current flows through D1 and the voltage drop across RL will be equal to the input voltage. During the negative half cycle of the input, the anode of D1 becomes negative and the anode of D2 becomes positive. Hence, D 1 does not conduct and D2 conducts. The load current flows through D2 and the voltage drop across RL will be equal to the input voltage. It is noted that the load current flows in the both the half cycles of ac voltage and in the same direction through the load resistance. ME2255/Electronics& Microprocessors Department of ME 7 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 8 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 9 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 10 www.Vidyarthiplus.com www.Vidyarthiplus.com 2. Discuss about intrinsic & extrinsic semiconductor. (AUC MAY 2012) ME2255/Electronics& Microprocessors Department of ME 11 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 12 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 13 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 14 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 15 www.Vidyarthiplus.com www.Vidyarthiplus.com ME2255/Electronics& Microprocessors Department of ME 16 www.Vidyarthiplus.com www.Vidyarthiplus.com 3.
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